Electron tube module and optical device

An electron tube module comprising:

• an electron tube;
• and
• a casing which accommodates the electron tube,
• wherein the electron tube includes:
• a vacuum container which includes a light transmitting substrate and forms a vacuum space;
• a photocathode provided in an inner surface corresponding to a surface on a side of the vacuum space of the light transmitting substrate and configured to emit photoelectrons into the vacuum space in response to the light incident through the light transmitting substrate;
• an electron detector provided inside the vacuum container and configured to detect electrons derived from the photoelectrons;
• and
• a prism bonded to an outer surface on a side opposite to the inner surface of the light transmitting substrate,
• wherein the prism includes:
• a first surface bonded to the outer surface of the light transmitting substrate;
• a second surface which is a light incident surface inclined with respect to the first surface;
• and
• a reflection portion configured to further reflect light incident to the photocathode through the prism and the light transmitting substrate and reflected at an interface between the photocathode and the vacuum space so that the light is incident to the photocathode again,
• wherein the casing includes a wall portion provided with an opening,
• wherein the second surface of the prism is parallel to the wall portion, and
• wherein at least a part of the second surface of the prism is exposed to outside through the opening.